Ventricular Tachycardia (VT) is a life-threatening condition characterized by an abnormally high rate of ventricular contraction. During VT, the ventricles lack sufficient time to fill with blood prior to each contraction often resulting in dizziness, loss of consciousness and sudden cardiac arrest. Catheter ablation has been shown to be an effective means for curing many arrhythmias, but current approaches are not able to coagulate tissue in the midmyocardium or subepicardial regions where foci responsible for VT often originate. We have developed a cooled-tip laser catheter (CTLC) capable of creating large lesions that extend into these regions with little to no thermal damage to the endocardium. In our phase I study we designed, built, and tested prototypes of the CTLC system. The current system is comprised of an 8F deflectable catheter, which houses a fiber optic and a pathway for circulation of saline. We incorporated a low cost pump system and a low-power diode laser to complete the system. Acute and chronic animal studies were performed to test the prototype system and the results were indeed dramatic. Using our CTLC system, we successfully produced large (1 cm in diameter) lesions that began on average 1 mm below the irradiated surface. These lesions were free of char or carbonization and well circumscribed by a distinct border separating the lesion form normal tissue. Additional advantages of our approach include the ability to monitor real-time electrophysiological activity during delivery of laser energy. In Phase II we plan to refine the current CTLC by including functional mapping electrodes and improving maneuverability. Animal studies are designed to characterize in a thorough manner the dose response for our system, compare it against current state of the art ablation technologies, and acquire data necessary for submission of an investigational device exemption from the FDA for clinical trials. PROPOSED COMMERCIAL APPLICATION: This research is specifically targeted towards the development of an improved laser-based catheter for treatment of VT. Cardiac arrhythmias including ventricular tachycardia (VT) and ventricular fibrillation (VF) are responsible for 400,000 cases of sudden death in the U.S. each year. Unlike other therapies, our catheter has potential for providing a curative means for patients who suffer from VT, and therefore could become the treatment of choice in such patients.